This invention relates to a process for forming a patternwise coated powder layer, and more particularly to an improved process for forming a stable patternwise coated powder layer as in the phosphor screen of a color picture tube where the phosphor is a powdery material.
Three kinds of phosphors each emitting red, green or blue light are dot-wise or stripe-wise coated on the inside surface of a face plate for color picture tube. According to the conventional process, the phosphor coating layer is formed as follows: first of all, a phosphor layer of a first color, for example, a layer of a mixture of green light-emitting phosphor and a photo-sensitive resin, is formed on the inside surface of the face plate. As the photo-sensitive resin, a mixture of polyvinyl alcohol and ammonium bichromate is usually used. The layer is formed usually by applying a mixture of a photo-sensitive resin solution and the phosphor to the inside surface of the face plate, and then drying the applied mixture. Then, the layer is exposed to ultra-violet rays through apertures of a shadow mask. The ultra-violet-irradiated position corresponds to an incident area of election beams for exciting the phosphor, that is, the position to which the phosphor is to be fixed. The photo-sensitive resin at the exposed area becomes insoluble and the whole layer at the area becomes insoluble thereby. Then, the layer is washed with a solvent, usually water, and only the part of layer which has become insoluble by exposure to the ultraviolet rays remains on the face plate, while other part of layer is dissolved off. Then, a layer of phosphor of a second color is formed by using, for example a mixture of blue light-emitting phosphor and the photo-sensitive resin in the same manner as above, and further a layer of phosphor of a third color is formed by using, for example, a mixture of red light-emitting phosphor and the light-sensitive resin in the same manner as a above.
As is evident from the foregoing description, a process for preparing a phosphor screen of color picture tube is complicated, and many repetitions of wet application, water washing and drying are required. Thus, simplification of the process has been keenly desired.
The present inventors proposed a process for forming a phosphor screen for color picture tube through more simplified steps than before (Japanese Laid-open Patent Application No. 53-126861; corresponding U.S. patent application Ser. No. 895,372; and corresponding DOS 2,815,894, which is based and established on a new finding that a photolytic product of an aromatic diazonium salt has an ability to accept powdery grains. The proposed process is to form a patternwise coated powder layer on a substrate surface, and is characterized by (1) a first step of applying a thin layer of an aromatic diazonium salt or a photo-sensitive composition containing an aromatic diazonium salt as a photo-sensitive component capable of becoming sticky by light exposure to a substrate surface, (2) a second step of subjecting the thin layer to a patternwise light exposure, thereby making the light-exposed part sticky, and (3) a third step of bringing the light-exposed thin layer in contact with powdery grains, thereby making the thin layer accept the powdery grains according to an ability of the thin layer to accept the powdery grains.
In the production of phosphor screen of color picture tube, said second step and third step are repeated for the phosphors of second and third colors to form a phosphor pattern of three colors.
However, the coated powder layers formed according to such a process are not mechanically strong, and can be relatively easily peeled off from the coated substrate especially by water washing. This is because deliquescent zinc chloride, etc., formed by photolytic reaction of photo-sensitive sticky material (aromatic diazonium salt), is contained as a binder in the coated powder layer, and the binder is water-soluble.
To solve this problem, a process for fixing the coated powder layer through contact with ammonia, etc. was also proposed, but the fixation by the proposed process has such a disadvantage that even the unexposed part is fixed thereby. That is, when ammonia fixation is carried out after the application of phosphor layer of first color in the production of color picture tube, the secondly exposed part has a lower ability to accept the powder of phosphor of second color, and only a very small amount of the phosphor is attached thereto, or no phosphor is attached at all. Thus, the ammonia fixation must be carried out after the phosphors of three colors have been all attached thereto. However, in that case, such a problem as color contamination appears. That is, when the second light exposure and application of the phosphor of second color are carried out before the phosphor of first color is sufficiently attached, a small amount of the phosphor of second color is attached to the position to which the phosphor of first color is to be attached, because of the still remaining ability of that position to accept the powder, and a color contamination develops. It was found that the color contamination could be prevented by sufficiently attaching the phosphor of first color thereto, etc., but it took much time in operation.